NoClassDefFoundError on JBoss5

Recently have been experiencing a strange problem on our JBoss5. After running our app for a while, the clients who call the EJB's start Throwing NoClassDefFoundError on some classes. After a restart, all is fine again for a while until other functions start returning NoClassDefFoundError. It seems totally random and a restart of the JBoss seems to cure the problem. This particular JBoss runs in a VM with 4GB of RAM and 2 CPU's and more than enough disk space (it has never has less than 5Gb free at any time). We have increased the Xmx and XMs to 2048 Mb and the permgen sweeping to 512Mb (ridiculousness I know). Intersetingly, the same install runs elsewhere on a VM with half the memory and Xmx/Xms/permgen settings with no problems whatsover. The Only differnce being that the last stable one is not any major load , although the broken one only has maximum of 8 clients connecting wh开发者_开发问答ich could hardly constitues "load" in my books :-). Has anybody come across this kind of problem, or have any idea of what it could be?

Not really an answer, be we had a RPM install for CENTOS 5. We removed that and used the zip file from the the jboss site instead. That cured the problem. Looks like we had a Dodgy install.